Blow molding of hollow articles is described, for example, in the commonly assigned copending application Ser. No. 09/132,350 which was filed Aug. 12, 1998 as a division of Ser. No. 08/758,490 (now U.S. Pat. No. 5,840,223, issued Nov. 24, 1998), filed Nov. 29, 1996, both of which are based upon German Patent Document 19544634 of Nov. 30, 1995. In that application, among other things, various systems of controlling the blow molding operation have been set forth.
In prior methods for extrusion blow molding of hollow bodies, especially for the production of canisters, containers, fuel receptacles for vehicles (gasoline tanks) and the like, it has been necessary to take into consideration the fact that the shape of the body may deviate from a cylindrical geometry.
In earlier systems the tubular preform has been extruded through an annular nozzle gap between a mandrel and a nozzle ring and the nozzle gap width and its geometry could be controlled in accordance with a wall thickness program and/or as a function of the length of the preform extruded from the gap.
For example, an axial movement of the mandrel and/or the nozzle ring could be generated in accordance with a first programming curve (WDS) of a wall thickness program to vary the gap width over the entire perimeter. The nozzle gap geometry could be varied in response to a second programming curve (RWDS) of the wall thickness program.
In the case of hollow bodies to be made by an extrusion blow molding process, where a cylindrical blow molded structure is to be formed, there is a sag of the preform based upon its own weight. The degree of sag is a function of the weight of the preform, the viscosity of the plastified synthetic resin material at the particular instantaneous preform temperature or the stiffness of that material, and the time for which the preform has been hanging. The sag can give rise to changes in the preform diameter and at least in upper regions thereof to a reduction in the wall thickness to the point that the preform will unintentionally tear away from the blow molding head. These effects can be compensated in part by enlargement of the gap width as a function of the extruded preform length. The control of the gap width here is effected in a accordance with a programming curve, also known as wall thickness control (WDS), which regulates the axial position of the mandrel or the nozzle ring and can vary the nozzle gap width over the entire perimeter of the gap. Each control operation is repeated with each cycle and takes place during the extrusion of the preform and hence has been termed a dynamic control.
In combination with the described dynamic width control, a dynamic control has also been effected and repeated for each machine cycle, of the nozzle gap geometry, based upon a second programming curve, also known as radial wall thickness control (RWDS). The second programming curve runs synchronously with the axial wall thickness control in the preform length direction and operates an effector or setting device which varies the gap geometry.
In practice a variety of techniques can be used to effect control in accordance with these two curves. Advantageously the dynamic control of the nozzle gap geometry can be carried out by adjusting either the inner side or wall of the nozzle gap thereof by displacing a deformable sleeve which delimits one side of the gap based upon the value given by the programming curve which is a function of time or extruded length, both being equivalent for a preform formed at a constant flow rate.
Auxiliary and radially adjustable sliders can be used which can be shifted into and out of the flow passage for the plastic melt based upon the RWDS programming curve value and within the blowing head or at the end of the nozzle. The dynamic regulation of the nozzle gap geometry can be used to generate thicker and thinner regions in the cross section of the preform, i.e. in the profile thereof so that, where the wall portions are stretched to a greater extent in blowing, i.e. at the corners, sufficient material will be provided to allow the production of the canisters and containers where the desired thickness of material and to permit formation of the bottoms and tops of such hollow bodies, at which additional material may be required beyond that for the formation of the walls. The shape variation permits the thick and thin regions to be shifted in the peripheral direction and to be a function of the gap width. A process describing this type of control is found in Plastverarbeiter 32, (1981) Number 3, pages 326-330.
In the extrusion blow molding of noncylindrical hollow bodies, hollow bodies with rectangular cross sections, like fuel tanks for Rotor vehicles and the like, preforms are extruded which have thick regions and/or thin regions with a certain distribution around the periphery and can extend over all or apart of the length of the preform. In conventional systems the nozzle gap geometry for this purpose can be adjusted manually. The setting can be effected, for example, by set screws which operate upon the deformable sleeve. The setting of the nozzle gap geometry in combination with an axial positioning of the mandrel and/or the nozzle ring for influencing the nozzle gap width over the entire perimeter thereof, does not, however, suffice when one must fabricate hollow bodies of uniform wall thickness which deviate significantly from a circular cross section.